1. Field of the Invention
The present invention relates generally to medical devices and, more particularly, to temperature control mechanisms and uses for a micro heat pipe catheter.
2. Description of the Related Art
Millions suffer from cancer, and new techniques for cancer treatment are continually being developed. The use of local hyperthermia (elevating the temperature of a cancerous pan of the body to a slightly higher temperature) has received increased attention over the past few years. Heating a cancerous tumor, including the edges of the tumor, to therapeutic temperatures of 42.5.degree. C. (108.5.degree. F.) to 43.0.degree. C. (109.4.degree. F.) for periods of 20 to 30 minutes will in most cases destroy the rapidly growing cancer cells and arrest tumor growth.
Total body temperatures above 41.8.degree. C. (107.2.degree. F.) are detrimental to the functions of the central nervous system, heart, liver, and kidneys, and may even cause histologically obvious damage to tissue cells, whereas tumorcidal effects are generally not observed below 42.5.degree. C. (108.5.degree. F.). At brain temperatures of over 41.8.degree. C. (107.2.degree. F.), the mechanism that regulates body temperature can become incapacitated, and there is danger of `malignant` or `runaway` hyperthermia. Further, temperatures of up to 45.degree. C. (113.0.degree. F.) may cause soft tissue necroses and fistulas as well as skin bums. Therefore, accurate temperature control of a localized area is critical to successful hyperthermia.
There is a significant need for development of a simple hyperthermia device which will generate a precisely controllable temperature. The heat should be confined to the diseased region to minimize the risk of damage to the surrounding normal tissue and to preserve normal bodily functions. Local hyperthermia should elevate the temperature of a cancerous tumor to a therapeutic level while maintaining the temperature of the surrounding tissue at, or near normal levels.
Numerous heating methods for tumor treatment have been proposed over the past few decades, and several methods are currently being practiced. These heating techniques may be classified from a clinical point of view as non-invasive and invasive.
Non-invasive hyperthermia techniques focus electromagnetic or ultrasonic energy on the region to be heated. This energy heats the body tissues to the desired temperatures. However, it is not possible to confine this energy to the diseased tissue, and the resulting effect is regional heating rather than local heating. Due to this regional heating, this technique often exhibits large temperature fluctuations due to variations in blood flow and thermal conductivity of the tissue. To better focus the energy to minimize regional heating, the wavelength of the energy beam must be small compared to the tumor's dimensions. An undesirable side effect of reducing the wavelength renders the technique useful for treating diseased areas only a few centimeters into the body. Another limitation is caused by bones being very strong absorbers of ultrasonic waves and air cavities being almost perfect reflectors. Bones may absorb a disproportionate amount of energy, and the reflections cause energy to disperse uncontrollably.
Invasive heating techniques, as compared with non-invasive techniques, are typically better for achieving therapeutic temperature levels without appreciable heating of normal tissues, regardless of the tumor geometry. Invasive heating techniques include the perfusion of the extremities with extracorporally heated blood and the irrigation of the urinary bladder with heated saline. Other invasive heating techniques include placing heating elements directly into the tumor. The use of a number of heating elements facilitates the regulation of temperature throughout the tumor.
Invasive hyperthermia devices include: (1) sets of implanted electrodes connected to a radio frequency generator; (2) combinations of implanted and external electrodes; (3) implanted microwave antennas; and (4) implanted or injected thermoseeds. Each of these invasive devices exhibit drawbacks.
The use of implanted electrodes, while simple, involves placing an array of needles into the tumor and connecting them to an RF generator. The temperature field for such electrodes is very difficult to control, and the volume that can be heated effectively is rather small, requiring many implants. Therefore, this technique is complicated, and the arrangement may result in non-uniform heating.
Electrodes require connections to a power source, and many electrodes are required to treat most tumors. The large number of connection wires or coaxial feed lines associated with the electrodes are cumbersome and may overheat.
Implanting microwave antennas or thermoseeds are probably the most popular invasive heating techniques. Generally, an array of antennas or thermoseeds is implanted in the tumor and left in place for the duration of the treatment. The antennas absorb externally-applied microwave energy, and the thermoseeds absorb externally-applied magnetic energy. Each antenna and thermoseed acts as a small heating unit, transferring heat to the tumor by conduction. The antennas and thermoseeds require careful placement in the tumor to optimize local heating of the tumor. This is particularly so with thermoseeds, because their orientation with respect to the induced magnetic energy determines their heating pattern. Furthermore, because all antennas and thermoseeds are heated to the same temperature by externally-applied energy, areas with poor blood flow may overheat while areas with high blood flow may not attain therapeutic temperatures.
The present invention is directed to overcoming or minimizing one or more of the problems discussed above.